Go's GC is not compacting, it has significant mutator overhead on every pointer write, it has an expensive allocator (it has to search through free lists, since it can't compact), and the free pahse has to do work proportional to the amount of garbage the program produces. It also produces no debugging information for ownership, for some bizarre reason (you can't tell from a Go heap dump with any standard Go tool which object is keeping another object around).

The fact that it can do this mostly concurrently (it's not fully concurrent, actually) is not that special - Java's latest GC is also mostly concurrent, without having any of the other previous drawbacks.

Go's big advantage in terms of memory management is that it doesn't produce as much garbage as most GC languages. The fact that it natively supports value types, and actually uses them extensively in the standard library (unlike C#, which also supports these), and it's excellent stack escape analysis which allows many allocations to stay on the stack and outside the GC heap entirely, is what makes Go have such little memory overhead.